Midway through the Hebeth mission in , you'll find a door blocked by fire shooting out of some broken pipes. Curious treasure hunters will want to know what's inside, but it's easy to overlook when you're racing from one fight to the next—this is one problem you can't solve with a meaty punch or shotgun.
The solution to this is hiding in plain sight, but to save you from sweeping the whole area multiple times like I did, here's what you need to do to turn off the fire blocking the door.
How to open the fire door in Doom: The Dark Ages' Hebeth mission
To disable the fire above the door, you need to walk past the door and climb up the wall to the concrete island in the middle of the area. Then, throw your shield at the nearby barricade to enter a locked-off room where you'll find a valve near the exit to turn off the fire. Once you've turned the wheel, the fire will be disabled, and you can walk into the secret area.
Given how small and inconspicuous the wheel you need to interact with is, I completely missed it the first time through, leading me to search high and low for a solution.
Your reward for opening the fire door is, sadly, rather boring. All you'll get is a life sigil and a handful of gold for your trouble, which, if you're playing on anything but the hardest difficulty, you've probably reached the maximum life sigil capacity of three at [[link]] any given time. This is what happened to me, so I actually had to leave the reward on the ground.